Specialty foams may be manufactured using a process that includes grinding materials, blending materials, extrusion, irradiation and foaming. This process may produce irradiation crosslinked polypropylene foam, which can be used for automotive interior trim applications, for example.
FIG. 1 shows auxiliary components used in a conventional process to manufacture irradiation crosslinked polypropylene foam. The process shown in FIG. 1 includes components for grinding materials, mixing a blend, extrusion, irradiation and foaming. Although any combination of these steps may be performed at a single location, they may also be performed at different locations to minimize costs. For example, manufacturing foam at the same production site where material ingredients are blended and extruded is often more costly than simply shipping extruded structures for subsequent foaming elsewhere. Extruded structures are more compact for transport to another site where foaming can be performed by, for example, an automobile manufacturer.
In step 100 of FIG. 1, resins are initially transported to a production site and fed to a grinder that pulverizes the resins into a powder form. The resins are formed as pellets, granules, chips, flakes, beads, cylinders, tubes, or the like, before being pulverized into a powder form. In step 102, powdered resins are stored in a raw materials silo along with other material ingredients. In step 104, the powdered resin and other material ingredients are blended together by using a Henschell type mixer. The other material ingredients may include an antioxidant package, a crosslinking agent, a blowing agent (i.e., foaming agent), and the like. In step 106, the blended material mixture is extruded into structures by using a die on a parallel twin screw extruder that has a short length to diameter (L/D) ratio, which minimizes shear and residence time. In step 108, an electron beam is used to produce irradiation crosslinked structures. The foaming process of step 110 includes pre-heating the irradiation crosslinked structure, followed by a salt bath that is used as a heat conducting medium to activate the blowing agent in the irradiation crosslinked structure to create foam. In step 112, the foam is cooled, washed, and wound.
Other conventional methods for manufacturing crosslinked foam may use chemical crosslinking, instead of radiation to produce irradiation crosslinked polypropylene or polyethylene foam. However, chemical crosslinking yields foam that may not be used for automotive interior trim applications because the foam is not smooth. A smooth surface is typically used for automotive interior trim because these applications include a bi-laminate of foam and a laminated foil of TPO or PVC. Consequently, chemical crosslinking is of limited use for these types of applications because it does not yield uniform foam cells. Other known methods include reactive extrusion, where a chemical reaction or crosslinking takes place during the extrusion process.
Accordingly, a need exists for manufacturing irradiated cross-linked foams in a low cost and efficient manner.